Surging engine - Any ideas???

So my 2004 3.4L started surging the other day. During mid-band, under acceleration I noticed a subtle surge in the engine (surging up and down constantly), until I backed off the gas. Well over the course of a few miles it got quickly worse - surging more and more at lower and lower speeds. Right before I pulled over, even when I floored it - it would make the lower guttural sound the Taco's do, but it was like it wasn't getting much gas.

Have about 80K miles on it - it's an automatic - been running perfect since I've had it the last few years.

I love my mechanic who has worked on Toyota's for 30 years or so - but after the first day they're having a problem figuring out what it is.

Any thoughts? Thanks!

 

Try cleaning the MAF first. No code?

 

IAC (Idle air control) valve maybe?

 

is it a "drive by wire" throttle by chance?

 

This? Had the same problem wit a 97 Acura. Replaced the IAC, problem solved.

 

Still checking, will look at these - thanks!!

 

The previous owner of my truck said he replaced the fuel pump because it was surging, and that fixed it. That was on a gen 2 but seems universal.

 

Yes, work your towards that as most of these problems have that behavior w/o throwing codes.

EDIT: You'll eventually start getting a 'lean' code from these but by then you may have pulled out all of your hair.

 

Under what conditions does it still surge now? Does it surge at idle and under power or only at specific rpms? Knowing that will help you start to look for problems.

 

I'm not a mechanic. PCV valve, the molded hose, grommet, etc., all good ?
Below from a mechanics forum, for Lexus, Toyota, etc.

If the PCV system is not functioning properly, the flow of crankcase vapor into the intake manifold will not be correctly metered. This, in turn, will upset the fuel/air mixture for combustion and can cause rough idling or even stalling of the engine. Furthermore, intake and exhaust valves, in addition to sparking plugs, may eventually be burned and rendered useless, prematurely affecting performance and requiring expensive repairs. To assure trouble-free performance of the PCV system and, in turn, the engine and vehicle, routine maintenance of the PCV system are recommended and required.

 

It has good fuel pressure, etc. Not sure why the IAC would affect anything above idle?

 

Well just found out the catalytic converter is bad - that was a big problem. Being replaced hopefully that was it - my mechanic is baffled as it didn't throw and codes - he'd never seen that happen before. Will see if that clears it all up.

 

I've see a couple of these on the YouTube mechanic's channel I like to watch, South Main Auto. Same thing - no power at high rpm's, idles great, no codes, but engine chokes and sputters from all the back pressure at higher rpms. He even had one like yours where it happened all of a sudden when someone was driving down the highway. He said the cat material can ball up and all of a sudden let go and get stuck in the exit end. He was able to diagnose it by doing a relative pressure test and see that the pressure in the cylinders on the exhaust stroke never went down to atmospheric.

 

I am not throwing codes. Having a surging at idle and everywhere else. Surges faster as I accelerate. Replaced throttle body, no change, did plugs n wires cleaned maf.

So your cat being bad caused this??

 

Yeah, but he wasn't idling rough.

 

Hmm. I am not running rough as much as pulsing type throttle once the engine starts to warm. Takes 40 seconds or so. At idle its like 200-300 rpms. Not sure what to do next.

 

It's idling that low? It should be 7-800 rpm. If there's no codes you can check to see if you have a cylinder that's not firing / making power by unplugging one spark plug at a time and seeing if that makes a difference. The cylinder that doesn't change how the engine is running will be the one that is misfiring if it feels like a single cylinder misfire.

Other than that, cleaning the MAF is a cheap solution that could cause rough running at all rpms.

 

Sorry, I meant the idle fluctuates 2-300 rpms every 1.5 seconds or so. The idle is at about 800. I have cleaned the MAF a few times. Surging is much more noticeable when under power.

 

Depending on how much experience you have, the maf can be tested to see if it's sending a good signal. You can test to see if the cat is plugged by doing a leakdown test or even just blowing compressed air a cylinder in the exhaust stroke and seeing if it blows through cleanly.

 

Turns out it was just the cc... mechanic had never seen that happen before - run like that anyway, no have it not throw a code when it was. Thanks everyone for the suggestions, etc!